1. Filed of the Invention
The present invention relates generally to the packaging of electronic components. More particularly, the present invention relates to a method of fabricating an integrated circuit package.
2. Description of the Related Art
Trends in modern integrated circuitry are generally towards smaller size and higher density. This has resulted in the requirement to fit a large number of bond pads (I/O pads) in a relatively small area of the integrated circuit, sometimes called the semiconductor chip.
To route electrical signals to and from the integrated circuit, the bond pads of the integrated circuit were electrically connected to traces on a substrate. However, since the traces were generally larger in size and pitch, i.e., spacing, than the bond pads, the number of traces which could be formed directly adjacent to the integrated circuit was limited.
One technique for overcoming the size and pitch limitations of the traces was to form the traces at a significant distance from the integrated circuit. However, this caused the length of the bond wires extending between the bond pads of the integrated circuit and the corresponding traces to be relatively long. Long bond wires were particularly problematic during the subsequent plastic encapsulation transfer molding process where the integrated circuit, the bond wires, and the substrate were transfer molded into a plastic encapsulant to form the finished integrated circuit package.
More particularly, during the plastic encapsulation transfer molding process, the plastic encapsulant was caused to flow around the integrated circuit, the bond wires, and substrate. This flow of plastic encapsulant against the bond wires caused: (1) the bond wires to become disconnected from the bond pads of the integrated circuit or from the traces; (2) caused the bond wires to break; and/or (3) caused the bond wires to move and short one another. This phenomena is called "wire sweep".
In the event a plastic encapsulation injection molding process was used, wire sweep was even more problematic. Generally, the plastic encapsulant was injected around the integrated circuit, the bond wires and the substrate at a higher pressure in a plastic encapsulation injection molding process than in a plastic encapsulation transfer molding process. Due to this higher pressure, more force was exerted against the bond wires, which were susceptible to wire sweep.
To avoid wire sweep, the bond wires were supported by an intermediate section between the bond pads of the integrated circuit and the traces.
Rostoker, U.S. Pat. No. 5,753,970, which is herein incorporated by reference in its entirety, teaches a lead support structure which supported a bond wire at a point along its length such that the bond wire was mechanically constrained and could not short to adjacent bond wires (see lead support structures 700a, 700b, 700c of FIGS. 9a, 9b, 9c, respectively, of Rostoker). However, providing and attaching such a lead support structure to the substrate was relatively labor-intensive and complex and thus significantly added to the cost of the integrated circuit package.
Typically, each trace was aligned with the specific bond pad on the integrated circuit to which the trace was to be connected. The bond pads on the integrated circuit were connected to the traces in the same order that the bond pads were position on the integrated circuit. However, in certain instances, it was desirable to allow for cross-over connections between the bond pads and the traces. Generally, a cross-over connection is a connection between a bond pad and a trace which is not directly aligned with the bond pad.
Gow, 3rd et al., U.S. Pat. No. 5,168,368, which is herein incorporated by reference in its entirety, teaches a bridge which was bonded to a first set of fingers of a lead frame. The bridge was located between the integrated circuit and a second set of fingers. To form a cross-over connection, a bond wire extending between a bond pad and a finger of the second set of fingers was intermediately bonded to the bridge above the first set of fingers. In this manner, the bond wire was crossed over fingers of the first set of fingers. However, providing and attaching such a bridge was relatively labor-intensive and complex and thus significantly added to the cost of the integrated circuit package.
Thus, in both Rostoker and Gow, 3rd et al., an intermediate bonding structure was provided between the bond pads and traces (fingers) to reduce the unsupported length of the bond wires and thus avoid wire sweep. However, the intermediate bonding structures had to be fabricated separately and attached to the package during assembly which, as set forth above, was relatively labor-intensive and complex and thus significantly added to the cost of integrated circuit package.